The present invention relates generally to cementitious compositions and, in particular, to gypsum-containing cementitious construction materials for high strength concrete, blocks, grout, floor underlayments, road-patching materials, backer boards, fiberboard and roofing tiles.
Ordinary Portland cement (OPC) is the main cementitious material of the building industry. It is the main binder in concrete blocks, roofing tiles, grouts, fiberboard, mortar, tile adhesives, etc. Disadvantages OPC include low early strength and high shrinkage on drying.
Gypsum (CaSO.sub.4.2H.sub.2 O) is an inexpensive, available material that is commonly used as an additive for OPC and for the production of gypsum-based products such as plasterboard, fiberboard, plaster, etc. Calcined gypsum, (calcium sulfate hemihydrate--CaSO.sub.4.1/2H.sub.2 O) forms gypsum upon wetting and sets within minutes, displaying excellent early-strength characteristics. The set gypsum has very low strength relative to OPC. In addition, the solubility of gypsum in water (2 g/l), along with high porosity, gives the material poor water-resistance.
Attempts to improve the water-resistance of gypsum boards by mixing Portland cement and calcined gypsum have met with limited success because of ettringite (3CaO.Al.sub.2 O.sub.3.CaSO.sub.4.32H.sub.2 O) formation. It is known that a major factor in the long-term deterioration of concrete is the formation of ettringite. This results from the reaction of tricalcium aluminate (3CaO.Al.sub.2 O.sub.3) present in OPC with sulfate. The formation of ettringite increases the volume of the concrete, causing splitting, cracking and crumbling. Although mixtures of OPC and calcined gypsum have previously been used and are in some cases advantageous, the interaction between the tricalcium aluminate and the sulfate has greatly limited the use of such mixtures. Their application has been primarily as "quick patches", which are known to be of poor long-term durability.
Much work has been focused on inhibiting the formation of ettringite. To this end, U.S. Pat. No. 3,852,051 deals with special formulations of Portland cement having low concentrations of tricalcium aluminate. Such formulations are expensive, however, and exhibit low ultimate compressive strength.
U.S. Pat. No. 4,494,990 to Harris discloses a cementitious composition containing OPC and alpha gypsum. The composition also includes a pozzolan source, such as silica fume, fly ash or blast-furnace slag. The Harris patent teaches that the pozzolan blocks the interaction between the tricalcium aluminate and the sulfate in the gypsum.
U.S. Pat. No. 4,661,159 discloses a floor underlayment composition that includes calcium sulfate alpha-hemihydrate (alpha gypsum), calcium sulfate beta-hemihydrate (beta gypsum), fly ash, and Portland cement. The patent also discloses that the floor underlayment material can be used with water and sand or other aggregate to produce a fluid mixture that may be applied to a substrate.
A cementitious composition useful for water-resistant construction materials is disclosed in U.S. Pat. No. 5,685,903 to Stav, et al. The composition includes beta gypsum, OPC, silica fume, and pozzolanic aggregate as filler. The OPC component may also contain fly ash and/or ground blast slag.
U.S. Pat. No. 5,685,903 teaches that the silica fume component is an extremely active pozzolan that prevents the formation of ettringite. It is further disclosed that compositions according to the invention that include both a pozzolanic aggregate and a finely divided pozzolan result in cementitious materials in which the transition zone between the aggregate and a cement paste is densified and thus produces a cured product of higher compressive strength than compositions which utilize a pozzolanic aggregate alone or a finely-divided pozzolan alone.
It is believed that the mechanism which causes changes in the microstructure of compositions according to the invention to result in higher compressive strengths is associated with two effects: 1) a pozzolanic effect in which the surfaces of the pozzolanic aggregate react with free lime to form calcium silicate hydrate (CSH) which becomes part of the product matrix: 2) a microfiller effect due to the fine size and spherical shape of the silica fume.
A cementitious binder composition useful for water-resistant, high-strength construction materials is disclosed by Stav, et al. in U.S. Pat. No. 5,858,083. The binder includes calcium sulfate beta-hemihydrate, a cement component comprising Portland cement, and either silica fume or rice-husk ash. The silica fume or rice-husk ash component is at least about 92% amorphous silica and has an alumina content of about 0.6 wt. % or less.
According to U.S. Pat. No. 5,858,083, the silica fume component is an extremely active pozzolan and prevents the formation of ettringite. The silica fume component includes at most 0.6 wt. % alumina in the form of aluminum oxide. U.S. Pat. No. 5,858,083 cites Malhotra, M., and Mehta, P. Kumar, Pozzolanic and Cementitious Materials, Advances in Concrete Technology, Vol. 1, who report typical oxide analyses of silicon fumes made from the ferrosilicon alloy industry having SiO.sub.2 amounts of as low as 83% and Al.sub.2 O.sub.3 amounts from between 1.00% and 2.5%. Oxide analyses of certain North American blast-furnace slags have SiO.sub.2 amounts of as low as 33% and Al.sub.2 O.sub.3 amounts as high as 10.8%. U.S. Pat. No. 5,858,083 concludes that not all pozzolans, and specifically, not all silica fumes, are acceptable for use according to the invention.
In all of the above-mentioned patents, methods have been developed for preventing the formation of ettringite. However, the prior art teaches that the formation of ettringite can actually be beneficial to cementitious material improving the compressive strength in the early stages.
The role of ettringite in expansive cements is related in U.S. Pat. No. 4,255,398. It is taught that ettringite should be precipitated on the surface of solids already present, and not from the liquid phase during cement hydration.
In Cement and Concrete Research, (Vol. 26, No.3), Singh and Garg report on the properties of a gypsum-based binder containing portland cement, calcined phosphogypsum, ground granulated slag and an organic retarder. The physical properties of the blended gypsum binder are compared with those of plain gypsum plaster. The superior behavior of the blended gypsum binder to water is attributed to the filling of voids and pores of the gypsum matrix with ettringite and CSH. The improvement of compressive strength of the blended gypsum binder over the 28-day test period is ascribed to the filling of the matrix with ettringite and tobermorite. The problems associated with late ettringite formation are not dealt with.
In Material Science of Concrete, Lawrence surveys and summarizes the topic of delayed ettringite formation (DEF). Based on extensive experimental investigations in the literature, the author teaches that correlation between expansions of siliceous sand mortars and the chemical composition of OPC indicate the importance of the sulfate level in the cement: where the sulfate level of a cement showing expansion has been increased by the addition of CaSO.sub.4 or Na.sub.2 SO.sub.4, the final expansion is increased. Added fly ash, blast furnace slag, or microsilica tends to reduce the expansions. This conclusion is supported by the work of Stav et al. (U.S. Pat. No. 5,858,083) for pozzolanic materials containing at most 0.6 wt. % alumina in the form of aluminum oxide. Higher levels of aluminum oxide promote DEF expansion, causing long-term deterioration of OPC.
U.S. Pat. No. 4,350,533 to Galer et al. discloses a cementitious composition containing high-alumina cement, calcium sulfate, and Portland cement and/or lime. The reaction is rapid, and the only significant factor contributing to strength during the very early stages of hydration (i.e., a few minutes to a few hours) is the formation of ettringite. Portland cement is not a necessary component of the composition and can be replaced by lime. A pozzolanic material such as montmorillonite clay, diatomaceous earth, pumice, and fly ash may be included in the cement powder as an optional ingredient. When used, it usually replaces part or all of the Portland cement.
High alumina cement, known also as Calcium Aluminate Cement, has an alumina content of 36-42%, the bulk of which is in the form of various calcium aluminates. Calcium aluminate cements containing high levels of sulfate are known for their susceptibility to DEF and to deterioration over the long-term. A commercial disclosure of LaFarge Fondu International A.S. reports that the addition of calcium sulfate to calcium aluminate cement should be limited to a maximum of 15-20% SO.sub.3 (25-34% calcium sulfate) to avoid excessive expansion which could disrupt the material.
U.S. Pat. No. 5,788,762 to Barger et al. discloses cementitious compositions comprised of gypsum (CaSO.sub.4.2H.sub.2 O), calcined clay and clinker. Novel methods of preparing these compositions are also disclosed. The pozzolanic material, calcined clay, has specified Fe and quartz contents, and contains kaolinites, montmorillonites, illites, halloysites, and mixtures thereof. The cementitious systems disclosed have a water demand of less than about 33%, one-day strengths of at least 1000 PSI, and low alkali functionality. However, the cementitious compositions reported are not fast-setting and have early compressive strengths that are comparable to those of ordinary Portland cements. U.S. Pat. No. 5.788,762 reports that an advantage of the novel cementitious system disclosed therein is that it allows for the addition of more gypsum than is normally added to the cement clinker, such that the calcium sulfate component amounts to 4-10 wt. % of the cementitious mixture.
It would be advantageous to formulate a material that is fast-setting, like that of U.S. Pat. No. 4,350,533 to Galer et al., but based on OPC. Calcium aluminate cement is expensive and has different chemical and physical properties from OPC, including a lower ultimate compressive strength and susceptibility to cracking and crumbling. It would be of further advantage to utilize controlled ettringite formation to improve the compressive strength of the cementitious material relative to known compositions, including those that block the formation of ettringite. It would also be of advantage to formulate a water-resistant material, which does not suffer from late-ettringite formation, such that the cement does not swell and crack over the long term, even in the presence of water. Finally, it would be of advantage to use significantly higher amounts of calcium sulfate in the cementitious mixture relative to prior art formulations, without compromising the ultimate compressive strength and without reducing the resistance to water.
U.S. Pat. No. 5,958,131 to Asbridge et al. discloses water-resistant cementitious compositions comprising calcium sulfate hemihydrate, portland cement and calcined clay, for use in applications in which water-resistance, good surface finish and a rapid gain in strength in the early stages following application are important.
It is taught therein that a hydrated mixture of calcium sulfate hemihydrate and portland cement might be expected to give the advantages of each of these two cementitious materials, however, deleterious chemical reactions occur between sulfate ions, which are supplied principally by the calcium sulfate, and aluminum compounds in the hydrated portland cement. For example, tricalcium aluminate and hydrated calcium aluminosulfate produce ettringite, a hydrated calcium aluminosulfate of large crystal volume. The expansive, forces introduced into a hardened cementitious product by the formation of ettringite can cause cracking and subsequent terminal deterioration of the product.
According to U.S. Pat. No. 5,958,131, the inclusion of calcined clay having a pozzolanic activity, e.g., metakaolin, in a cementitious composition, together with calcium sulfate hemihydrate and portland cement unexpectedly and beneficially is effective in rendering the composition more resistant to attack by water. While not wishing to be bound by any particular theory, the authors utilize well-known and established theory and attribute the water resistance to pozzolanic activity that consumes free lime and prevents the formation of ettringite. It is maintained by U.S. Pat. No. 5,958,131 to Asbridge et al. that the water resistance is achieved because of the reactivity of the calcined clay towards chemical compounds such as hydroxides of calcium and sodium and sulfates of calcium and sodium, which are produced during the hydration of mixtures of calcium sulfate hemihydrate and portland cement. Calcined clays such as metakaolin react with and immobilize chemical compounds that would otherwise take part in a reaction to form ettringite, which would cause expansion and deterioration of the hydrated hydraulic composition after setting.
It is further disclosed by U.S. Pat. No. 5,958,131 that properly hydrated hydraulic compositions in accordance with the invention exhibit, unexpectedly and beneficially, good workability in the wet state, develop strength in a relatively short time after setting, produce cast articles with good surface finish, and can have good long term stability and resistance to attack by water.
The role of the calcined clay according to U.S. Pat. No. 5,958,131 is the reactivity with hydroxides and the like, such that the formation of ettringite is prevented. This is strikingly similar to U.S. Pat. No. 4,494,990 to Harris, described above, which discloses a cementitious composition containing OPC, calcined gypsum, and a pozzolan source, such as silica fume, fly ash or blast-furnace slag.
The authors of U.S. Pat. No. 5,958,131 do not relate to the amorphous alumina component of calcined clay and its role in the chemical/physical development of the cement and in the physical properties of the cementitious material (strength, water resistance, etc.). This is also evident from the index of performance for metakaolin and other calcined clays that has been chosen and defined in said patent: reactivity with lime (the pozzolanic reaction). As both siliceous and aluminous materials react with lime, theoretically speaking, the calcined clay could contain solely siliceous or solely aluminous material. No ratio of siliceous/aluminous material is defined.
Moreover, U.S. Pat. No. 5,958,131 to Asbridge et al. claims water-resistance over an extremely wide range of percentages and ratios of OPC, hemihydrate, and calcined clay. The cementitious compositions disclosed that are deemed suitable for adding to water to form a water-resistant hydraulic solid composition comprise from 20% to 98% by weight of calcium sulfate hemihydrate, from 1% to 50% by weight of portland cement, and from 1% to 30% by weight of calcined clay having pozzolanic activity (e.g., metakaolin). The preferred ratio of hemihydrate to OPC is in the range of 2:1 to 10:1; the preferred ratio of OPC to calcined clay is in the range of 2:1 to 10:1. The proportion of calcium sulfate hemihydrate is preferably in the range of from 47.5% to 91% by weight, the proportion of portland cement is preferably in the range of from 7% to 40% by weight, and the proportion of calcined clay is preferably in the range of from 2% to 12.5% by weight.